A common means of specifying a value for a time varying signal, often referred to as an alternating current (AC) signal, whether the signal is in the form of a time varying voltage or in the form of a time varying current, is to use the root-mean-square (RMS) value of that signal. Less expensive AC meters, used to measure RMS values, often employ a technique wherein the meter responds to the average of the rectified signal but applies a scale factor in order to obtain an indication of the root-mean-square (RMS) value of the AC signal it is detecting. Such an xe2x80x9caverage respondingxe2x80x9d meter gives a correct value for the RMS voltage of a signal only for a sinusoidal signal of a single frequency which is free of significant distortion. These conditions are often not met which results in an incorrect RMS value indicated for the signal.
More accurate and more expensive AC meters used for measuring the RMS value of a signal employ an RMS converter. In the RMS converter, analogue circuitry first rectifies the time varying signal and then squares the resultant rectified values. Finally additional analogue circuitry in the RMS converter is used to average the squared values to obtain the RMS value of the signal. This resultant RMS value is the output signal of the RMS converter and will typically comprise a large direct current (DC) signal component and a smaller AC component. The value of the AC component, referred to as a ripple component, is controlled by the interaction of repetitive and non-repetitive components of the input signal, as well as the time constant of the RMS converter. Typically the output of the RMS converter is allowed to track the input at low input frequencies. The output of the RMS converter may be digitized by an analog-to-digital converter (A/D converter) at a rate high enough to capture any significant ripple coming out of the RMS converter. The output of the A/D converter can be fed into additional RMS computational circuitry which can be implemented digitally by a microprocessor.
In representative embodiments, a root-mean-square (RMS) meter includes an RMS converter having a converter input and a converter output. The RMS converter converts a time varying signal applied to the converter input to a signal at the converter output. The value of the signal at the converter output is indicative of the RMS value of the applied signal. The signal at the converter output comprises a non-time varying component and a time varying component as determined by a time constant of the RMS converter. The RMS meter further includes an inverting amplifier having an inverter input and an inverter output. The converter output is connected to the inverter input. In addition, the RMS meter includes a switch having first, second, and central contacts. The converter output is connected to the first contact, and the inverter output is connected to the second contact. When the switch is in a first position, the first contact is connected to the central contact, and when the switch is in a second position, the second contact is connected to the central contact.
A representative method for processing an input signal includes obtaining a root-mean-square signal by taking the root-mean-square of the input signal, inverting the root-mean-square signal, and using equal duty cycles, combining the root-mean-square""signal and the inverted root-mean-square signal by periodically switching between the two signals.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.